This invention is directed to oriented heat sealable polyolefin film structures possessing a low film to film coefficient of friction, excellent anti-static behavior and good optical clarity. This invention is also directed to multi-layer film structures having improved appearance and machine handling in the packaging of diverse items of commerce.
Highly crystalline polypropylene film is an excellent packaging material, but it has high film to film coefficient of friction which makes it difficult to utilize in automatic packaging equipment. Certain innovations have improved on the surface friction characteristics of highly crystalline polypropylene in film form. For example, in U.S. Pat. No. 3,176,021, incorporated herein by reference, it is taught to include minor quantities of fatty acid amides into the polypropylene. In order to obtain the benefits taught by this patent, however, certain limitations must be observed. The film must be formed from a melt and extruded at a temperature between about 400.degree.-550.degree. F. In addition, the amide must be present in amounts of from 0.005 to about 2.0 weight percent of the polypropylene and it must be present along with from about 0.1 to about 4.0 weight percent polyethylene. Under these conditions and limitations the resulting polypropylene film has static coefficients of friction which are significantly higher than present day requirements. In addition, such a film does not have the high stereoregularity required by present day packaging demands. Further, it has been found that once such films have been subjected to the high temperatures which are experienced in practice such as during heat sealing, i.e., approaching 270.degree. F., the coefficient of friction increases significantly and is nowhere near the present day requirements of about 0.25.
In U.S. Pat. No. 3,399,156, incorporated herein by reference, it is taught that the inclusion of the combination of silicone oil and finely divided silica into a polypropylene film produces a film which has a coefficient of friction of from 0.45 to 0.46 and is suitable for use in bread-wrapping machines. This is not a low enough coefficient of friction for use in present day, high speed automatic packaging machinery, however.
It has been found that to a certain extent high crystallinity in polypropylene impedes facile migration of additives to the surface of such a polymer in film form. This problem has been addressed in U.S. Pat. No. 4,419,410, incorporated herein by reference. This publication teaches that by the coextrusion of a skin layer of polypropylene of comparatively low stereoregularity with a core layer of polypropylene of comparatively high stereoregularity, containing a surface modifying agent such as an amide of a fatty acid, the amide will bloom to the surface of the layer having the comparatively low stereoregularity polymer with comparative ease.
U.S. Pat. No. 4,419,411, incorporated herein by reference in its entirety, significantly advanced the art of making multi-layer polypropylene films suitable for most high speed packaging operations. This patent discloses a multi-layer polypropylene film structure having surface modifying agents comprising finely divided silica, silicone oil and an amide of a water insoluble mono-carboxylic acid having from about 8 to about 24 carbon atoms on a surface thereof.
U.S. Pat. No. 4,590,125, incorporated by reference in its entirety, discloses a multi layer heat sealable polypropylene film structure comprising an amide-containing polypropylene base layer and two heat sealable skin layers of a low stereoregularity polyolefin and microcrystalline wax.
Nevertheless, despite the advances in film packaging technology as exemplified by the above referenced patents, their films suffered from one or more shortcomings. For example, some of the films, when used in high speed cigarette package wrapping machines, have a tendency for the film surfaces of contacting packages to stick together, particularly in areas where heat sealing has occurred.
It is an object of the present invention to provide an oriented, heat sealable polyolefin, preferably polypropylene film possessing a low coefficient of friction (COF), excellent anti-static properties and good optical clarity.
It is a further object to provide such films having a reduced tendency for the film surfaces to stick together when used in high speed packaging machinery, especially at locations where heat sealing has occurred.
It is another object to improve the aesthetic features of multi-layer films by improving uniformity and avoiding levels of additives which cause haze.
Yet another object is to secure improvements in the handling characteristics of films in accordance with the invention, to improve roll conformance and avoid film corrugation.
It is another object to provide a process for preparing multi-layer film structures in accordance with the invention.
These and other objects will be apparent from a review of the instant specification.